In most uses of silver halide in photographic materials, it is desirable to increase the speed or sensitivity of the emulsion. There are a number of different techniques for increasing the speed of an emulsion which are usually classified as chemical sensitization or spectral sensitization. Chemical sensitization usually involves modification of the silver halide grains to make the most efficient use of the radiation that they absorb. The three general types of chemical sensitization are sulfur sensitization, reduction sensitization, and precious (noble) metal sensitization. These methods of chemical sensitization are well known and firmly established in the art (e.g., James, T. H. and Vanselow, W. "Chemical Sensitization", J Photo. Sci., 1, 133 (1953), Freiser, H. and Ranz, E., Ber. der Bunsengesellschaft, 68, 389 (1964), and Pouradier, J. "Chemical Sensitization", Photographic Theory: Liege Summer School, A. Hautot, p. 111, Focal Press (London 1963).
Spectral sensitization enables grains to benefit from radiation in regions of the electromagnetic spectrum where the silver halide would ordinarily not absorb. Dyes which absorb radiation and can transfer energy to the grains to help in the photoreduction of silver ions to clusters of silver metal are conventionally used to effect spectral sensitization.
Another phenomenon associated with the use of spectral sensitizing dyes is known in the art as supersensitization. The addition of other substances, frequently in quantities ranging from less than an equivalent molar rate to a 100 fold molar excess of supersensitizer to dye, can increase the spectrally sensitized speed of the emulsion by more than an order of magnitude. Some supersensitizers are dyes themselves, but many others do not absorb radiation in significant amounts in the visible portion of the electromagnetic spectrum. Therefore, the effect of supersensitizers on spectral sensitization is not clearly dependent on the ability of compounds to absorb radiation in the visible portion of the spectrum. Certain cyanines, merocyanines compounds analogous to cyanines, certain acylmethylene derivatives of heterocyclic bases, and ketone derivatives such as p-dimethylaminobenzalacetone are known supersensitizers. An expanded selection of supersensitizers is therefore desired.
Silver halide emulsions can be protected against the production of fog and stabilized against the loss of sensitivity during keeping. Suitable antifoggants and stabilizers which can be used alone or in combination, include triphenylphoshines, amines, arsines, bismuthines, and stibildynes taught in U.S. Pat. No. 4,578,347; the thiazolium salts described in Staud, U.S. Pat. No. 2,131,038 and Allen, U.S. Pat. No. 3,694,716; the azaindenes described in Piper, U.S. Pat. No. 2,886,437 and Heimbach, U.S. Pat. No. 2,444,605; the mercaptotetrazoles described in Kendall et al., U.S. Pat. No. 2,403,927, Kennard et al., U.S. Pat. No. 3,266,897 and Luckey et al., U.S. Pat. No. 3,397,987; and the oximes described in Carroll et al., British Pat. No. 623,448. With respect to infrared sensitive emulsions, G.B. Pat. No. 2,140,928 describes the use of heterocyclic indolinium dyes to maintain stability.
However, it is well known in the prior art that infrared sensitive photographic materials exhibit poor keeping properties (Neblette's Handbook of Photography and Reprography, 7th. Edition, Ed. Sturge, Pub. Van Nostrand). Even with the above mentioned additives, it has not been possible to maintain adequate stability of an infrared sensitive emulsion. As a consequence, many infrared sensitive photographic materials stipulate refrigeration, and put limitations on storage humidity in order to extend the shelf-life as much as possible.
By incorporating quaternary aryl- or alkyl- Group V compounds, in particular the tetraphenylphosphonium salts, into an infrared sensitive emulsion, we have found that photographic speed may be increased, the liquid emulsion may be held for extended times prior to coating, and the keeping properties of the material improved.